This invention relates to mode transition control of a direct injection spark ignition engine.
In direct injection spark ignition engines, the engine operates with stratified air/fuel operation in which the combustion chamber contains stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures.
The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during the intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to conventional three-way catalytic converters to reduce CO, HC, and NOx. When operating at air/fuel mixtures lean of stoichiometry, a NOx trap or catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx.
The stratified mode of operation is typically utilized when the engine is operating in light to medium loads. The homogeneous mode of operation is typically used from medium to heavy load operating conditions. In certain conditions, it is necessary to transition from one engine mode of operation to the other. During these mode transitions, it is desired to deliver the requested engine output torque to provide good drive feel. However, in some circumstances, the range of acceptable lean air/fuel ratios of stratified operation do not overlap with the acceptable air/fuel ratios of homogeneous operation. Therefore, during the mode transition, a torque shock occurs because of the step change in engine air/fuel ratio.
One method of preventing the engine torque disturbance during mode transition is to change the injection mode one cylinder at a time according to the required amount of fuel to be injected. This reduces a large torque disturbance to several smaller torque disturbances. Such a method is described in U.S. Pat. No. 5,170,759.
The inventors herein have recognized a disadvantage with the above approach. Even though the large torque jump during mode transition is avoided, there are still several smaller torque jumps experienced. In other words, a single, large torque disturbance is substituted with multiple smaller torque disturbances which are still noticeable by the vehicle driver.
An object of the invention herein is to control an engine during mode transitions to provide a smooth torque output.
The above object is achieved and disadvantages of prior approaches overcome by a method for controlling an engine during a cylinder air/fuel ratio change from a first cylinder air/fuel ratio to a second cylinder air/fuel ratio. The method comprising the steps of determining a first number of cylinders to enable to perform combustion so that the cylinder air/fuel ratio change can occur without a disturbance in engine torque, enabling said first number of cylinders and changing the cylinder air/fuel from the first cylinder air/fuel ratio to the second cylinder air/fuel ratio when said number of cylinders are currently disabled, and disabling a second number of cylinders and changing the cylinder air/fuel ratio from the first cylinder air/fuel ratio to a third cylinder air/fuel ratio otherwise.
By using cylinder activation as an additional degree of freedom, abrupt changes in average engine torque can be avoided even during air/fuel ratio changes. Thus, during a mode transition where engine air/fuel ratio is constrained to change due to, for example, combustion limits, abrupt changes in average engine torque can be avoided by changing a number of active cylinders. Also, a check is made as to whether the required number of cylinders to activate are currently deactivated.
An advantage of the above aspect of the invention is that abrupt changes in engine output torque can be avoided during mode transitions.
Another advantage of the above aspect of the invention is that the range of stratified operation can be extended since the range of available transitions is increased.
Yet another advantage of the above aspect of the invention is that emissions can be reduced since the engine can be operated farther from air/fuel ratio combustion limits.
In another aspect of the invention, the above object is achieved by a method for controlling an engine during a mode transition from homogeneous operation to stratified operation, wherein a cylinder air/fuel ratio changes from a first cylinder air/fuel ratio to a second cylinder air/fuel ratio. The method comprising the steps of determining a first number of cylinders to enable to perform combustion so that the cylinder air/fuel ratio change can occur without a disturbance in engine torque, enabling said first number of cylinders and changing the operating mode from homogeneous mode to stratified mode when said number of cylinders are currently disabled, said enabling step further comprising changing cylinder air/fuel from the first cylinder air/fuel ratio to the second cylinder air/fuel ratio, and disabling a second number of cylinders and changing the cylinder air/fuel ratio from the first cylinder air/fuel ratio to a third homogeneous cylinder air/fuel ratio otherwise.
By disabling cylinders when it is determined that cylinder activation is required yet there are insufficient cylinders to activate, it is possible to perform mode transitions using cylinder activation.
An advantage of the above aspect of the invention is that abrupt changes in engine output torque can be avoided during mode transitions.
Another advantage of the above aspect of the invention is that the range of stratified operation can be extended since the range of available transitions is increased.
Yet another advantage of the above aspect of the invention is that emissions can be reduced since the engine can be operated farther from air/fuel ratio combustion limits.